io_uring/rsrc: optimise single entry advance

Iterating within the first bvec entry should be essentially free, but we
use iov_iter_advance() for that, which shows up in benchmark profiles
taking up to 0.5% of CPU. Replace it with a hand coded version.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/rsrc.c

@@ -1364,7 +1364,10 @@ int io_import_fixed(int ddir, struct iov_iter *iter,
 		const struct bio_vec *bvec = imu->bvec;
 
 		if (offset <= bvec->bv_len) {
-			iov_iter_advance(iter, offset);
+			iter->bvec = bvec;
+			iter->nr_segs = bvec->bv_len;
+			iter->count -= offset;
+			iter->iov_offset = offset;
 		} else {
 			unsigned long seg_skip;